This disclosure is related to a heat treatment method and to components heat treated according to the method.
Superalloys are metallic alloys for elevated temperature service, generally based on group VIIA elements of the periodic table, and are used for elevated temperature applications where resistance to deformation and stability are desired. The common superalloys are based on nickel, cobalt or iron. Nickel-iron base superalloys such as, for example Alloy 706 are generally employed as materials of construction in gas turbine engine components such as rotor discs (hereinafter rotors) and spacers.
Nickel-iron base superalloys such as Alloy 706 are generally employed as materials of construction in gas turbine engine components such as rotor discs (hereinafter rotors) and spacers. As a result of the demand for improved performance and efficiency, the components of modern gas turbine engines operate near the limit of their properties with respect to temperature, stress, and oxidation/corrosion. Due to these aggressive operating environments, the superalloy materials from which the components are made must possess a combination of exceptional properties including high strength capabilities at elevated temperatures and rotational speeds. In particular, it is desirable for nickel-iron base superalloy articles suitable for components such as turbine rotors and discs to possess resistance to crack growth.
There are two known heat treatment processes that are prescribed by International Nickel Company (INCO), the inventor of the Alloy 706. The two known heat treatment processes are heat treatment A and heat treatment B respectively. Heat treatment A is recommended for optimum creep and high temperature rupture properties, while heat treatment B is recommended for applications requiring high tensile strength.
Heat treatment A comprises a solution treatment at 1700 to 1850° F. for a time commensurate with the section size, followed by a first air cooling. The first air cooling is followed by a stabilization treatment at 1550° F. for three hours, followed by a second air cooling. Following the second air-cooling is a precipitation treatment at 1325° F. for 8 hours. The object is then cooled in a furnace at a rate of 100° F./hr to 1150° F. where it is held for 8 hours. The cooling in the furnace is followed by a third air cooling.
Heat treatment B comprises a solution treatment at 1700 to 1850° F. for a time commensurate with the section size followed by a first air cooling. The first air cooling is followed by a precipitation treatment at 1325° F. for 8 hours followed by cooling in a furnace at a rate of 100° F./hr to a temperature of 1150° F. where it is held for 8 hours. This is followed by a second air cooling.
In general, heat treatment A is recommended for optimum creep and rupture properties, while heat treatment B is recommended for applications requiring high tensile strength. It is generally desirable for a turbine rotor to display high tensile strength at low and intermediate temperatures (of less than or equal to about 700° F.) in some locations. High tensile strength is generally desirable in parts near the bore and bolt-holes while optimum creep behavior is desirable in other parts such as, for example, near the radially outer end of turbine rotor wheel or disk. However, the radially outer end is generally at higher temperature during operation. If heat treatment A is used, the strength at the bore is not adequate, and if heat treatment B is used, there is not enough creep resistance at the high temperatures. As a result, surface flaws or cracks can propagate rapidly under stress at temperatures above 900° F.
The cracks can occur due to one or more mechanisms. One such mechanism is hold time fatigue cracking. This mechanism generally occurs when the turbine rotor is subjected to extensive operation under high temperatures and high stress at temperatures above 900° F. To prevent such cracking, the turbine rotor has to be frequently visually inspected. This increases down-time as the turbine has to be shut down and dissembled. In addition, the visual inspection may not detect all cracks. This method of crack prevention is generally not suited to power production turbines.
Another method of crack prevention comprises using inlet conditioning schemes to reduce compressor discharge temperature. These inlet conditioning schemes generally use lower turbine temperatures. These lower temperatures however, degrade gas turbine performance.
It is therefore desirable to provide a heat treatment for components manufactured from superalloys such as, for example, Alloy 706 that facilitates an improvement in hold time fatigue crack growth resistance.